GB-2227071 discloses a method for installing a plastics service pipe in an existing steel service pipe which extends from a main pipe to a meter within a consumer's premises. Firstly the meter is removed: then the plastics pipe is inserted through a gland fitted to the open end of the existing service pipe and advanced along the service pipe up to a point adjacent the junction of the service pipe with the main pipe. The plastics pipe has a sealing head fitted to its leading or forward end, which closes that end of the pipe and also seals against the interior of the existing service pipe. Once the plastics pipe has been advanced to the required position, fluid sealant is fed into the annular space between the exterior of the plastics pipe and the interior surface of the existing service pipe. The sealant is allowed to set, and then a flexible rod having a pointed end is advanced along the plastics pipe to break the seal across its forward end and open the replacement pipe for the flow of gas from the main pipe.
The above-described method is applicable to low pressure systems, in which therefore the force acting on the sealing head, as the plastics pipe is advanced into the existing service pipe, is relatively small: accordingly, the plastics pipe can be advanced manually without difficulty. However, for higher pressure systems, for example systems operating at pressures up to 7 bar, the resulting force acting on the sealing head at the forward end of the plastics pipe would be too high to permit the plastics pipe to be advanced manually along the existing service pipe.
GB-2299645 discloses methods for installing a replacement plastics pipe in an existing steel service pipe, which methods are suited to use on systems which operate at high pressures. In these methods, a seal is advanced along the existing service pipe using an elongate flexible assembly, but the seal is not deployed until it reaches the required position. Some embodiments require the seal to be advanced beyond the far end of the existing service pipe, where it joins the main pipe, and then deployed to engage against that end of the service pipe: however, it is not always possible or convenient to achieve this and the seal may not be wholly effective, for example if the end of the existing pipe is damaged. Other embodiments require that the elongate flexible assembly advances the plastics replacement pipe along the existing service pipe at the same time as it advances the seal into position, and therefore before the seal is deployed: however, this can lead to difficulties in advancing the replacement pipe against the full normal operating pressure.